Does decimate an object helps cycles rendering faster in blender?
Sure. Just like always in computer science: the more data you have, the longer it takes to calculate and the more memory it takes as well. However, it's not that much as you'd probably expect since the lighting needs to be calculated anyway.

Original Geometry vs. Decimated Geometry

Quick comparison of rendering a high-poly model (3.519.529 faces) and its  decimated version (35.224 faces ~1%) in a static scene using a simple backdrop and 3 area lights:


Render Time: 10:07.97 | Faces: 3.519.529  | Memory: 447.2 Mb


Render Time: 07:37.17 | Faces: 35.224 | Memory: 6.5 Mb